A long-awaited inquiry into claims that British troops murdered and tortured civilians during the Iraq War is to begin on Monday.

The Al-Sweady Inquiry is examining claims that UK soldiers murdered 20 or more Iraqis and tortured detainees after the "Battle of Danny Boy" in Maysan Province, southern Iraq, in May 2004.

The inquiry is named after 19-year-old Hamid Al-Sweady, one of those who died after a firefight between UK soldiers and insurgents at a checkpoint named Danny Boy on May 14, 2004.

It will look into allegations of unlawful killing at UK base Camp Abu Naji on May 14 and 15 2004, and alleged ill-treatment of five Iraqi nationals at the camp and at a detention facility at Shaibah Logistics Base in Iraq between May 14 and September 23 that year.

The allegations have been vigorously denied by the Ministry of Defence (MoD).

The inquiry will be the second examination of the case after an earlier Royal Military Police inquiry was judged inadequate by high court judges following a legal battle by Mr Al-Sweady's uncle and five Iraqis who claim they were abused by British troops after the firefight.

Since it was ordered in 2009, the inquiry team has trawled through mountains of evidence, as well as taking statements from Iraqi witnesses in Beirut and Istanbul, as well as from military witnesses.

Some 15 Iraqis will travel to the UK to give evidence to the inquiry later this month, including Hamid Al-Sweady's uncle Khuder Al-Sweady as well as several detainees.

A further 45 Iraqis, including relatives and medical staff will give evidence via videolink from the British Embassy in Beirut, Lebanon.

Evidence from military witnesses is hoped to start around September, continuing until February next year, and the inquiry report is hoped to be finished by the end of 2014.

In his opening, set to take several days, Jonathan Acton Davis QC. is expected to give a comprehensive outline, including why the inquiry was ordered and what it aims to identify.

He will discuss the allegations of unlawful killing, as well as allegations concerning the detention of detainees at Camp Abu Naji, and what happened when they were moved to Shaibah Logistics Base.

The inquiry will look at, but not criticise, an original Royal Military Police investigation into what happened, secretary Cecilia French said last week.

After Mr Acton Davis' opening statement, the inquiry will hear from counsel representing the Iraqi complainants; military witnesses; and the Ministry of Defence.

Attorney General Dominic Grieve has agreed to grant witnesses to the inquiry immunity from prosecution based on their own evidence.

The inquiry will open this morning at Finlaison House, central London.